rtc: tps6594: introduce private structure as drvdata

This patch will prepare for the next one (power management support) by
introducing struct tps6594_rtc.

No functionnal change.

Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Link: https://lore.kernel.org/r/20240618141851.1810000-3-richard.genoud@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>